Tunable illuminator for lithography systems

ABSTRACT

In one example, an apparatus includes an extreme ultraviolet illumination source and an illuminator. The extreme ultraviolet illumination source is arranged to generate a beam of extreme ultraviolet illumination to pattern a resist layer on a substrate. The illuminator is arranged to direct the beam of extreme ultraviolet illumination onto a surface of a photomask. In one example, the illuminator includes a field facet mirror and a pupil facet mirror. The field facet mirror includes a first plurality of facets arranged to split the beam of extreme ultraviolet illumination into a plurality of light channels. The pupil facet mirror includes a second plurality of facets arranged to direct the plurality of light channels onto the surface of the photomask. The distribution of the second plurality of facets is denser at a periphery of the pupil facet mirror than at a center of the pupil facet mirror.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/928,236, filed Oct. 30, 2019, which is hereinincorporated by reference in its entirety.

BACKGROUND

Extreme ultraviolet (EUV) lithography is an optical lithographytechnique in which the scanner uses light in the extreme ultravioletregion (e.g., spanning wavelengths of approximately one to one hundrednanometers). A light source is configured to emit EUV radiation. Forinstance, the light source may vaporize a molten metal such as tin intoa highly ionized plasma that emits the EUV radiation. The EUV radiationis subsequently guided, using a series of optics (e.g., includingmultilayer mirrors), into the scanner. In the scanner, the EUV radiationis used to project a pattern, which is etched into a photomask, onto asemiconductor wafer. The EUV process can be used to fabricate a highresolution pattern of features onto the semiconductor wafer, potentiallyat a scale of seven nanometers or beyond.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates the concept of off-axis illumination as employed inexamples of the present disclosure;

FIG. 2 is a simplified schematic diagram of an example lithographysystem, according to examples of the present disclosure;

FIG. 3 is a top view of an example of the pupil facet mirror of FIG. 2 ,according to examples of the present disclosure; and

FIG. 4 illustrates a flowchart of a method of fabricating asemiconductor device according to at least one embodiment of the presentdisclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In one example, the present disclosure provides a tunable illuminatorfor lithography systems, including extreme ultraviolet (EUV) lithographysystems. As discussed above, an EUV lithography system may use EUVradiation to project a pattern formed in a photomask onto asemiconductor wafer, and the pattern may then be etched into the wafer.The EUV process can be used to fabricate a high resolution pattern offeatures onto the semiconductor wafer, potentially at a scale of sevennanometers or beyond for the features' critical dimensions. The criticaldimensions may be controlled by controlling the exposure energy of thelithography system (which varies the size of the feature dimensions) andthe depth of focus (DOF) at the semiconductor wafer. Together, theexposure energy and the DOF may define a process window during whichfeatures having the critical dimensions may be printed on thesemiconductor wafer.

One technique that has emerged as an effective way to increase theprocess window for lithography systems is freeform source optimization.Freeform source optimization involves modifying the intensity of eachsource pixel, so that a flexible, freeform source shape can be definedfor each layer of the semiconductor wafer that is being printed.However, in EUV lithography systems (which typically have more complexillumination systems than non-EUV lithography systems), designlimitations of the illuminator make it more challenging to adjust theintensity of the source pixels to achieve effective freeform sourceoptimization.

Examples of the present disclosure provide a tunable illuminator forlithography systems, including EUV lithography systems. In one example,the tunable illuminator includes a pupil facet mirror whose facets arearranged in a grid pattern. The cells of the grid (e.g., the facets)vary in size and shape, and the distribution of the cells over the gridis similarly non-uniform. In one particular example, the density of thecells at the outer edges of the grid pattern (e.g., at the outer edgesof the pupil facet mirror) is greater than the density of the cells atthe center of the grid pattern (e.g., at the center of the pupil facetmirror).

Within the context of the present disclosure, the “effective area” of aprojection lens is understood to refer to the area of the projectionlens in which a target feature pitch can be resolved. In other words, toresolve the target feature pitch, the light from the illuminator shouldbe angled to be incident upon the projection lens in the effective area.The effective area varies with the size of the target feature pitch.Typically, as the target feature pitch shrinks, so does the effectivearea (although the relationship between the pitch and the effective areais not necessarily linear).

In one example, the illuminator of the present disclosure may bearranged to illuminate the effective area using off-axis illumination(OAI). In this case, the light directed by the illuminator onto thephotomask is incident upon the photomask at a direction that is notnormal to the surface of the photomask (i.e., an angle between the beamof light and the photomask surface is oblique). In turn, the diffractionpattern of the photomask shifts within the objective of the projectionlens. When the photomask grating has a periodic or regular shape, thephotomask's diffraction pattern comprises a plurality of diffractionorders (e.g., points of light) into which the light may be split. Thesmaller the pitch of the grating, the fewer the number of diffractionorders that passes through the projection lens.

FIG. 1 , for instance, illustrates the concept of off-axis illuminationas employed in examples of the present disclosure. As illustrated, abeam of light 100 may be directed from an illuminator 102 and onto asurface of a photomask 104. The angle θ at which the beam of light 100is incident upon the surface of the photomask 104 may be oblique (i.e.,non-parallel to an imaginary line A-A′ that is normal to the surface ofthe photomask 104). As a result, the −1, 0, and +1 diffraction orders(n) pass through the photomask 104; however, only the 0 and −1diffraction orders are incident upon (and, thus, collected by) theprojection lens 106. The light from the 0 and the −1 diffraction orderssubsequently interfere with each other to form a steady image on thesemiconductor wafer 108. Other (higher) diffraction orders may exist(e.g., +2, −2, +3, −3, and so on), but may be lost due to the sizelimits of the optical system.

By contrast, if the beam of light 100 struck the photomask 104 at anangle of ninety degrees relative to the photomask surface (e.g., suchthat the beam of light 100 was parallel to the line A-A′), then all ofthe diffraction orders would be shifted equally in a manner that bringsthe 0 diffraction order parallel to the line A-A′. As a result, the +1and −1 diffraction orders might not be collected by the projection lens106 (e.g., both of the +1 and −1 diffraction orders would fall outsidethe area of the projection lens 106, as illustrated by the dashed linesin FIG. 1 ).

When the beam of light 100 strikes the photomask 104 at an angle of θ,the angle between adjacent diffraction orders (e.g., between the −1 and0 order, and between the 0 and +1 order) is equal to the angle θ.Typically, the greater the value of θ (and, more specifically, thelarger the angle between the adjacent diffraction orders), the greaterthe number of source points that is formed at the edges of theprojection lens 106. Thus, as θ increases, so does the illumination ofthe projection lens's effective area. As a result, smaller pitchfeatures can be printed on the semiconductor wafer 108. Put another way,the larger the angle between the adjacent diffraction orders that arecollected by the projection lens 106, the smaller the features that canbe printed on the semiconductor wafer 108. This relationship may bequantified by:

$\begin{matrix}{{\sin\;\theta} = \frac{n \times \lambda}{pitch}} & \left( {{EQN}.\mspace{14mu} 1} \right)\end{matrix}$where n is the diffraction order and λ is the wavelength of the lightthat is incident upon the photomask 104.

Thus, OAI may be employed in examples of the present disclosure toenhance the resolution of the patterns that are printed ontosemiconductor wafers under given numerical apertures (NAs) for theprojection optics. The NA may be defined by the convergent angle betweenthe projection lens and the semiconductor wafer. For instance, NA may bedefined as:NA=r×sin θ(EQN. 2)

Where r represents the refractive index between the final projectionlens and the semiconductor wafer (e.g., r=1 for air, 1.43 for waterimmersion, etc.). Thus, NA represents the ability of the projection lens106 to collect diffracted light. To achieve the finest resolution (e.g.,the smallest pitch) using OAI, the maximum incident angle between theilluminator and the photomask would be determined by the NA value. In alithographic system having the photomask 4× larger than thesemiconductor wafer image, the NA of the illuminator would be equal tothe NA of the projection lens divided by four. Generally, the higher theNA if the projection lens 106, the bigger the projection lens 106 is,the better the resolution of the features that can be printed is, andthe lower the DoF of the projection optics.

Additional features can be added to the illuminator disclosed herein.Some of the features described below can also be replaced or eliminatedfor different examples. Although some examples disclosed below discussoperations that are performed in a particular order, these operationsmay be performed in other orders as well without departing from thescope of the present disclosure.

Moreover, the illuminator and methods disclosed herein may be deployedin a plurality of applications, including the fabrication of fin-typefield effect transistors (finFETs). For instance, examples of thepresent disclosure may be well suited for patterning the fins of afinFET to produce a relatively close spacing between features. Infurther examples, spacers used in forming the fins of the finFET may beprocessed according to examples of the present disclosure.

FIG. 2 is a simplified schematic diagram of an example lithographysystem 200, according to examples of the present disclosure. Thelithography system 200 may also be referred to herein as a “scanner”that is operable to perform lithography exposing processes withrespective radiation sources and exposure modes.

In one example, the lithography system 200 generally comprises ahigh-brightness light source 202, an illuminator 204, a mask stage 206,a photomask 208, a projection optics module 210, and a substrate stage212. In some examples, the lithography system may include additionalcomponents that are not illustrated in FIG. 2 , such as gas supplymodules, exhaust modules, and/or other components. In further examples,one or more of the high-brightness light source 202, the illuminator204, the mask stage 206, the photomask 208, the projection optics module210, and the substrate stage 212 may be omitted from the lithographysystem 200 or may be integrated into combined components.

The high-brightness light source 202 may be configured to emit radiationhaving wavelengths in the range of approximately one nanometer to 250nanometers. In one particular example, the high-brightness light source202 generates EUV light with a wavelength centered at approximately 13.5nanometers; accordingly, in some examples, the high-brightness lightsource 202 may also be referred to as an “EUV light source.” However, itwill be appreciated that the high-brightness light source 202 should notbe limited to emitting EUV light. For instance, the high-brightnesslight source 202 may be utilized to perform any high-intensity photonemission from excited target material.

In one example, the term “approximately” is understood to mean +/−twentypercent of the stated value, and more typically +/−ten percent of thestated value, and more typically +/−five percent of the stated value,and more typically +/−three percent of the stated value, and moretypically +/−two percent of the stated value, and more typically +/−onepercent of the stated value, and even more typically +/−0.5 percent ofthe stated value. The stated value is therefore an approximate value. Inthe absence of any specific description, any stated value stated hereinis approximate in accordance with the above definition.

In some examples (e.g., where the lithography system 200 is a UVlithography system), the illuminator 204 comprises various refractiveoptical components, such as a single lens or a lens system comprisingmultiple lenses (zone plates). In another example (e.g., where thelithography system 200 is an EUV lithography system), the illuminator204 comprises various reflective optical components, such as a singlemirror or a mirror system comprising multiple mirrors. The illuminator204 may direct light from the high-brightness light source 202 onto themask stage 206, and more particularly onto the photomask 208 that issecured onto the mask stage 206. Thus, the illuminator 204 focuses andshapes the radiation produced by the high-brightness illumination source202 along the light path, in order to produce a desired illuminationpattern upon the photomask 208.

In an example where the high-brightness light source 202 generates lightin the EUV wavelength range, the illuminator 204 comprises reflectiveoptics, such as a field facet mirror 218, a pupil facet mirror 220, andvarious relay mirrors (not shown). As discussed in further detail below,each of the field facet mirror 218 and the pupil facet mirror 220 maycomprise a plurality of reflective facets (e.g., the field facet mirrormay include a first plurality of reflective facets, while the pupilfacet mirror may include a second plurality of reflective facets).Broadly, EUV radiation may be collected from the high-brightness lightsource 202 and focused as a beam onto the field facet mirror 218, wherethe facets of the field facet mirror split the beam into a plurality oflight channels. The plurality of light channels is reflected towardcorresponding facets of the pupil facet mirror 220, which forms imagesof the facets of the field facet mirror 218. The relay mirrors (e.g.,conic relay mirrors) may subsequently direct the images of the facets ofthe field facet mirror onto the plane of the photomask 208.

The mask stage 206 may be configured to secure the photomask 208. Insome examples, the mask stage 206 may include an electrostatic chuck(e-chuck) to secure the photomask 208. This is because the gas moleculesabsorb EUV light, and the lithography system 200 for EUV lithographypatterning is maintained in a vacuum environment to minimize EUVintensity loss. Herein, the terms “photomask,” “mask,” and “reticle” maybe used interchangeably. In one example, the photomask 208 is areflective mask.

In some examples, the photomask 208 may comprise a reflective mask. Oneexample structure of the photomask 208 includes a substrate formed froma suitable material, such as a low thermal expansion material (LTEM) orfused quartz. In various examples, the LTEM may include TiO₂, dopesSiO₂, or other suitable materials with low thermal expansion. Thephotomask 208 may further include a reflective multilayer deposited onthe substrate.

The reflective multilayer may include a plurality of film pairs, such asmolybdenum-silicon (Mo/Si) film pairs (e.g., a layer of molybdenum aboveor below a layer of silicon in each film pair) in certain examples.Alternatively, the reflective multilayer may includemolybdenum-beryllium (Mo/Be) film pairs, or other suitable materialsthat are configurable to highly reflect the EUV light. The photomask 208may further include a capping layer, such as a capping layer formed fromruthenium (Ru), disposed on the reflective multilayer for protection.The photomask 208 may further include an absorption layer, such as atantalum boron nitride (TaBN) layer, deposited over the reflectivemultilayer. The absorption layer may be patterned to define a layer ofan integrated circuit (IC). Alternatively, another reflective layer maybe deposited over the reflective multilayer and may be patterned todefine a layer of an integrated circuit, thereby forming an EUV phaseshift mask.

In some examples, a pellicle 214 may be positioned over the photomask208, e.g., between the photomask 208 and the substrate stage 212. Thepellicle 214 may protect the photomask 208 from particles and may keepthe particles out of focus, so that the particles do not produce animage (which may cause defects on a wafer during the lithographyprocess).

The projection optics module 210 may be configured for imaging thepattern of the photomask 208 onto a semiconductor wafer 216 secured onthe substrate stage 212, e.g., by projecting a patterned beam ofradiation onto the semiconductor wafer 216. In one example, theprojection optics module 210 comprises refractive optics (such as for aUV lithography system). In another example, the projection optics module210 comprises reflective optics (such as for an EUV lithography system).The light directed from the photomask 208, carrying the image of thepattern defined on the photomask 208, may be collected by the projectionoptics module 210. The illuminator 204 and the projection optics module210 may be collectively referred to as an “optical module” of thelithography system 200.

In some examples, the semiconductor wafer 216 may be a bulksemiconductor wafer. For instance, the semiconductor wafer 216 maycomprise a silicon wafer. The semiconductor wafer 216 may includesilicon or another elementary semiconductor material, such as germanium.In some examples, the semiconductor wafer 216 may include a compoundsemiconductor. The compound semiconductor may include gallium arsenide,silicon carbide, indium arsenide, indium phosphide, another suitablematerial, or a combination thereof. In yet another example, thesemiconductor wafer 216 may include an alloy semiconductor, such assilicon germanium, silicon germanium carbide, gallium arsenic phosphide,or gallium indium phosphide. In other examples, the semiconductor wafer216 may comprise a silicon-on-insulator (SOI) or agermanium-on-insulator (GOI) substrate. The SOI substrate may befabricated using a separation by implantation of oxygen (SIMOX) process,a wafer bonding process, another applicable process, or a combinationthereof.

In some examples, the semiconductor wafer 216 comprises an undopedsubstrate. However, in other examples, the semiconductor substrate 216comprises a doped substrate, such as a p-type substrate or an n-typesubstrate.

In some examples, the semiconductor wafer 216 includes various dopedregions (not shown) depending on the design requirements of thesemiconductor device structure. The doped regions may include, forexample, p-type wells and/or n-type wells. In some examples, the dopedregions are doped with p-type dopants. For example, the doped regionsmay be doped with boron or boron fluoride. In other examples, the dopedregions are doped with n-type dopants. For example, the doped regionsmay be doped with phosphor or arsenic. In some examples, some of thedoped regions are p-doped and other doped regions are n-doped.

In some examples, an interconnection structure may be formed over thesemiconductor wafer 216. The interconnection structure may includemultiple interlayer dielectric layers, including dielectric layers. Theinterconnection structure may also include multiple conductive featuresformed in the interlayer dielectric layers. The conductive features mayinclude conductive lines, conductive vias, and/or conductive contacts.

In some examples, various device elements are formed in thesemiconductor wafer 216. Examples of the various device elements mayinclude transistors (e.g., metal oxide semiconductor field effecttransistors (MOSFETs), complementary metal oxide semiconductor (CMOS)transistors, bipolar junction transistors (BJTs), high-voltagetransistors, high-frequency transistors, p-channel and/or n-channelfield effect transistors (PFETs and/or NFETs), diodes, or other suitableelements. Various processes may be used to form the various deviceelements, including deposition, etching, implantation, photolithography,annealing, and/or other applicable processes.

The device elements may be interconnected through the interconnectionstructure over the semiconductor wafer 216 to form integrated circuitdevices. The integrated circuit devices may include logic devices,memory devices (e.g., static random access memory (SRAM) devices), radiofrequency (RF) devices, input/output (I/O) devices, system-on-chip (SoC)devices, image sensor devices, other applicable devices, or acombination thereof.

In some examples, the semiconductor wafer 216 may be coated with aresist layer that is sensitive to EUV light. Various componentsincluding those described above may be integrated together and may beoperable to perform lithography exposing processes.

It will be appreciated that FIG. 2 represents a simplified form of alithography system 200. In some examples, the lithography system 200 mayinclude additional components that are not illustrated, such asadditional optics, a plasma source, and other components.

FIG. 3 is a top view of an example of the pupil facet mirror 220 of FIG.2 , according to examples of the present disclosure. As illustrated, thepupil facet mirror may have a generally circular shape and may comprisea plurality of facets 300 ₁-300 _(m) (hereinafter individually referredto as a “facet 300” or collectively referred to as “facets 300”)arranged in the generally circular shape. It should be noted that forthe sake of simplicity, only a few of the facets 300 are labeled in FIG.3 .

In some examples, the facets 300 are arranged in a grid pattern, asshown. Thus, the facets 300 may also be referred to as “cells” of thegrid pattern. As shown in FIG. 3 , the distribution of the facets 300 onthe pupil facet mirror 220 may be non-uniform. More specifically,although the distribution of the facets 300 may be locally uniform overlocal areas of the pupil facet mirror 220 (e.g., facets 300 that areequidistant from the center of the pupil facet mirror 220 may bedistributed in a uniform manner relative to each other), thedistribution of the facets 300 is globally non-uniform (i.e.,non-uniform over the entire surface of the pupil facet mirror 220). Forinstance, the density of the grid pattern may increase from the centerof the grid pattern to the periphery of the grid pattern (e.g., alongthe directions of the arrows 302). Consequently, the sizes of the facets300 may decrease from the center of the grid pattern to the periphery ofthe grid pattern. In other words, the facets 300 that are positionedaround the periphery of the pupil facet mirror 220 (e.g., such as facet300 _(m) and its neighboring facets 300) may be smaller and more denselypacked than the facets 300 that are positioned closer to the center ofthe pupil facet mirror 220 (e.g., such as facet 300 ₁ and itsneighboring facets 300).

The facets 300 may take any shape. Moreover, the shapes and/ordimensions of the facets 300 may vary over the area of the pupil facetmirror 220. That is, the facets 300 do not necessarily all have the sameshape and/or dimensions. For instance, in the example illustrated inFIG. 3 , the facets 300 closest to the center of the pupil facet mirror220 (e.g., including facet 300 ₁) are wedge-shaped; however, the facets300 that are further away from the center of the pupil facet mirror 220(e.g., including facets 300 ₂-300 _(m)) may be shaped more like curvedpolygons (e.g., rectangles, trapezoids, or the like). Moreover, thesizes and dimensions of these curved polygons may vary. For instance,the curved polygons of the facets 300 that are closest to the peripheryof the pupil facet mirror 220 may be the smallest (i.e., have thesmallest respective areas). Thus, the facets 300 may include facets ofat least two different sizes and/or at least two different shapes. Itshould be noted that although the facets 300 are illustrated as havingcertain shapes and arrangements in FIG. 3 , the facets 300 may be shapedand arranged in manners other than what is illustrated, as long as thedistribution of the facets 300 is denser at the periphery of the pupilfacet mirror 220 than at the center of the pupil facet mirror.

The arrangement of facets 300 on the pupil facet mirror 220 ensuresgreater illumination in the effective areas of a projection lens. This,in turn, allows greater illumination (e.g., a greater number ofdiffraction orders) to be collected in the effective areas (e.g., at theedges) of the projection lens, as described above. As a result of thegreater illumination in the effective areas, features with smallerpitches may be resolved on a semiconductor wafer.

FIG. 4 illustrates a flowchart of a method 400 of fabricating asemiconductor device according to at least one embodiment of the presentdisclosure. At least some steps of the method 400 may be performed via acontroller of an EUV lithography system, such as the lithography systemillustrated in FIG. 2 .

While the method 400 is illustrated and described below as a series ofacts or events, it will be appreciated that the illustrated ordering ofsuch acts or events are not to be interpreted in a limiting sense. Forexample, some acts may occur in different orders and/or concurrentlywith other acts or events apparat from those illustrated and/ordescribed herein. In addition, not all illustrated acts may be requiredto implement one or more aspects or embodiments of the descriptionherein. Further, one or more of the acts depicted herein may be carriedout in one or more separate acts and/or phases.

The method 400 begins in step 402. In step 404, an EUV light source maybe activated to pattern a resist layer on a substrate (where thesubstrate may be a semiconductor wafer). For example, the EUV lightsource may be part of a lithography system such as the systemillustrated in FIG. 2 and discussed above. Thus, the EUV light sourcemay generate light in extreme ultraviolet wavelengths (potentially byvaporizing droplets of metal into a highly ionized plasma).

In step 406, light emitted by the EUV light source may be directed ontoa photomask, using an illuminator that includes a plurality ofreflective facets arranged in a non-uniform grid pattern. In oneexample, the plurality of reflective facets is arranged as a pupil facetmirror of the illuminator. In another example, the non-uniform gridpattern may vary such that the facets are distributed more densely atthe periphery of the pupil facet mirror than at the center of the pupilfacet mirror. For instance, the plurality of facets may be arranged asshown in FIG. 3 . Direction of the light by the illuminator describedabove may cause the light to strike the surface of the photomask atoblique angles.

In step 408, a plurality of diffraction orders of the light that passthrough the photomask may be collected by a projection lens in theeffective areas of the projection lens. The effective areas of theprojection lens may be defined relative to a target pitch for featuresto be printed on the substrate. For instance, as discussed above, thesmaller the target pitch is, the smaller the corresponding effectivearea for printing the target pitch will be.

In step 410, the light may be directed from the projection lens onto theresist layer, to pattern the resist layer. For instance, exposure of theresist layer to the light may cause features having the target pitch tobe printed in the resist.

In one embodiment, the blocks 404-410 may be continuously repeatedduring operation of the EUV light source (e.g., for multiple layers ofthe substrate). At block 412, the method 400 may end.

Thus, examples of the present disclosure enable features with very smallpitches to be printed on semiconductor wafers by an EUV lithographysystem using a freeform source. In one example, the present disclosureprovides an apparatus that includes an extreme ultraviolet illuminationsource and an illuminator. The extreme ultraviolet illumination sourceis arranged to generate a beam of extreme ultraviolet illumination topattern a resist layer on a substrate. The illuminator is arranged todirect the beam of extreme ultraviolet illumination onto the surface ofa photomask. In one example, the illuminator includes a field facetmirror and a pupil facet mirror. The field facet mirror includes a firstplurality of facets arranged to split the beam of extreme ultravioletillumination into a plurality of light channels. The pupil facet mirrorincludes a second plurality of facets arranged to direct the pluralityof light channels onto a surface of the photomask. The distribution ofthe second plurality of facets is denser at a periphery of the pupilfacet mirror than at a center of the pupil facet mirror.

In another example, an extreme ultraviolet light source is activated topattern a resist layer on a substrate. Light emitted by the extremeultraviolet light source is directed onto a photomask using anilluminator that includes a plurality of reflective facets arranged in anon-uniform grid pattern. A plurality of diffraction orders of the lightthat pass through the photomask are collected in an effective area of aprojection lens. Light of the plurality of diffraction orders is thendirected onto the resist layer.

In another example, the present disclosure provides an apparatus thatincludes a high-brightness light source, an illuminator, a photomask,and a projection lens. The high-brightness light source is arranged togenerate illumination to pattern a resist layer on a substrate. Theilluminator directs the illumination onto a surface of the photomaskusing a plurality of reflective facets arranged in a non-uniform gridpattern. The photomask generates a plurality of diffraction orders fromthe illumination. The projection lens collects at least some diffractionorders of the plurality of diffraction orders and focuses the at leastsome diffraction orders onto the resist layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An apparatus, comprising: an extreme ultravioletillumination source arranged to generate a beam of extreme ultravioletillumination to pattern a resist layer on a substrate; and anilluminator arranged to direct the beam of extreme ultravioletillumination onto a surface of a photomask, wherein the illuminatorcomprises: a field facet mirror comprising a first plurality of facetsarranged to split the beam of extreme ultraviolet illumination into aplurality of light channels; and a pupil facet mirror, comprising asecond plurality of facets arranged to direct the plurality of lightchannels onto the surface of the photomask, and wherein a distributionof the second plurality of facets is denser at a periphery of the pupilfacet mirror than at a center of the pupil facet mirror, the secondplurality of facets comprises a first facet having a first sizepositioned at the periphery of the pupil facet mirror and a second facethaving a second size positioned at the center of the pupil facet mirror,the first size is smaller than the second size, the first facet has twoopposite linear edges both extending towards a center point of the pupilfacet mirror, the second facet has an arc edge with two ends, one of thetwo opposite linear edges of the first facet further extends towards aportion of the arc edge of the second facet between the two ends of thearc edge, and another one of the two opposite linear edges of the firstfacet is substantially aligned with a linear edge of the second facetpositioned at the center of the pupil facet mirror.
 2. The apparatus ofclaim 1, further comprising: the photomask, wherein the photomaskcomprises a grating to split each light channel of the plurality oflight channels into a plurality of diffraction orders; and a projectionlens to collect at least some diffraction orders of the plurality ofdiffraction orders and to focus the at least some diffraction ordersonto the resist layer.
 3. The apparatus of claim 2, wherein the secondplurality of facets is arranged to direct the plurality of lightchannels so that at least some light channels of the plurality of lightchannels are incident on the surface of the photomask at an angle thatis oblique relative to the surface of the photomask.
 4. The apparatus ofclaim 1, wherein the pupil facet mirror has a circular shape.
 5. Theapparatus of claim 1, wherein the first and second facets have differentshapes.
 6. The apparatus of claim 1, wherein the first facet ispositioned at an outermost position of the periphery of the pupil facetmirror, the second plurality of facets further comprises a third facetpositioned between the first and second facets, and the third facet hasa size larger than the first facet and smaller than the second facet. 7.A method, comprising: activating an extreme ultraviolet light source topattern a resist layer on a substrate; directing light emitted by theextreme ultraviolet light source onto a photomask using an illuminatorthat includes a plurality of reflective facets arranged in a non-uniformgrid pattern, the non-uniform grid pattern comprises a plurality ofcells, and each cell of the plurality of cells corresponds to a facet ofthe plurality of reflective facets, wherein a first cell and a secondcell of the plurality of cells are positioned at a periphery of thenon-uniform grid pattern, a third cell of the plurality of cells ispositioned at a center of the non-uniform grid pattern, a first edge ofthe first cell, a second edge of the second cell, and a third edge ofthe third cell are substantially coterminous, a fourth edge of the firstcell, a fifth edge of the second cell, and a sixth edge of the thirdcell are offset from each other, the first edge of the first cell isopposite to the fourth edge of the first cell, the second edge of thesecond cell is opposite to the fifth edge of the second cell, and thethird edge of the third cell is opposite to the sixth edge of the thirdcell; collecting a plurality of diffraction orders of the light thatpass through the photomask in an effective area of a projection lens;and directing light of the plurality of diffraction orders onto theresist layer.
 8. The method of claim 7, wherein a distribution of theplurality of cells is denser at the periphery of the non-uniform gridpattern than at the center of the non-uniform grid pattern.
 9. Themethod of claim 7, wherein the non-uniform grid pattern has a circularshape.
 10. The method of claim 7, wherein the first and third cells havedifferent sizes.
 11. The method of claim 7, wherein the first and thirdcells have different shapes.
 12. The method of claim 7, wherein thedirecting the light onto the photomask comprises directing the light sothat at least some beams of the light are incident on a surface of thephotomask at an angle that is oblique relative to the surface of thephotomask.
 13. The method of claim 7, wherein the plurality of cells arearranged to be a plurality of rings.
 14. The method of claim 7, whereina size of the second cell is larger than a size of the first cell andsmaller than a size of the third cell.
 15. An apparatus, comprising: alight source arranged to generate illumination to pattern a resist layeron a substrate; a photomask to generate a plurality of diffractionorders from the illumination; an illuminator to direct the illuminationonto a surface of the photomask using a plurality of reflective facetsarranged in a non-uniform grid pattern, wherein the non-uniform gridpattern comprises a plurality of cells, and each cell of the pluralityof cells corresponds to a facet of the plurality of reflective facets, afirst cell of the plurality of cells at a center of the non-uniform gridpattern is sector-shaped, and a second cell of the plurality of cells ata periphery of the non-uniform grid pattern has an arc edge shorter thanan arc edge of the first cell; and a projection lens to collect at leastsome diffraction orders of the plurality of diffraction orders and tofocus the at least some diffraction orders onto the resist layer. 16.The apparatus of claim 15, wherein the plurality of cells includes cellsof at least two different shapes.
 17. The apparatus of claim 15, whereinthe light source is an extreme ultraviolet light source.
 18. Theapparatus of claim 15, wherein the plurality of reflective facets isarranged to direct the illumination so that at least some of theillumination is incident on the surface of the photomask at an anglethat is oblique relative to the surface of the photomask.
 19. Theapparatus of claim 15, wherein the second cell of the plurality of cellsis a curved polygon.
 20. The apparatus of claim 15, wherein the firstcell has two opposite first linear edges, a third cell of the pluralityof cells has a size the same as a size of the second cell, the firstcell, the second cell, and the third cell are adjacent to each other,wherein the second cell has a second linear edge aligned to one of thetwo opposite first linear edges of the first cell, and the third cellhas a third linear edge aligned to another one of the two opposite firstlinear edges of the first cell.